The invention relates generally to a superconducting current limiting device, and more particularly to a superconducting current limiting device that uses magnetic field to achieve transition of superconductor from a superconducting state to a normal resistive state.
Fault current limiting in an AC electric power system can be accomplished by introducing variable impedance of a fault current limiter (FCL) into the power system during a fault to limit the magnitude of the fault current. Superconductors can provide such variable impedance by transitioning from their superconducting state to a normal resistive state where they exhibit ohmic resistive behavior. Superconductors remain in a superconductive state as long as their temperature, current density and magnetic field remain below critical levels, denoted as Tc, Jc and Hc. This characteristic is graphically illustrated in FIG. 1, where the region between the origin and the critical surface defined by Tc, Jc and Hc is the region where the superconducting material enters a superconducting state. Outside of the critical surface, the material becomes non-superconducting and exhibits resistive characteristics.
Because of the high surge current generated during a fault, the current in a current limiting device exceeds the critical current level of its superconducting element causing the material to transition from a superconducting state to a normal resistive state. This transition, referred to as xe2x80x9cquenching,xe2x80x9d is characterized by a nonlinear increase in the effective resistance defined as the voltage across the superconductor element divided by current. It is also known that the addition of magnetic field during this transition results in an additional increase in resistance until the effect saturates out at higher temperatures. One can think of the phenomena as the layering of xe2x80x9ciso-resistancexe2x80x9d shells that extend outward from the critical Tc-Jc-Hc surface in FIG. 1 until a saturation resistance is achieved. Therefore, in addition to the resistance that is achieved by current exceeding the critical current level, additional resistance can be achieved with application of a magnetic field to the superconductor.
The quenching of a superconductor to the normal resistive state and subsequent recovery to the superconducting state corresponds to a reversible xe2x80x9cvariable impedancexe2x80x9d effect. A superconducting device with such characteristics is ideal in a current limiting application. Such a device can be designed so that under normal operating conditions the peak operating current level is always below the critical current level of the superconductor elements. As such, effectively no power loss (I2R loss) will result during normal operation. When the fault condition occurs, however, the fault current level exceeds the critical current level of the superconducting elements, thus causing the device to enter a quench state. By the same token, mechanisms altering the device""s operating temperature and/or applied magnetic field level may be adapted as a catalyst to help achieving fast and uniform quenching of such a superconducting device. As long as the superconducting material is not damaged during quench, it can recover back to its superconducting state if the level of the above-mentioned three factors were reduced to within their respective critical level after quench.
U.S. Pat. No. 6,664,875, assigned to the assignee of the present invention, and entitled xe2x80x9cMatrix-type Superconducting Fault Current Limiter,xe2x80x9d herein incorporated by reference, describes a mechanism that combines all three of the quenching factors of a superconductor, namely current, magnetic field and temperature, to achieve more uniform quenching of the superconductor during current limiting. This MFCL uses a magnetic field as one of the triggering mechanisms to force fast and uniform quenching in the superconductor, therefore reducing burnout risks caused by non-uniformity in the superconductor material.
Methods and apparatuses to generate magnetic fields in superconducting current limiting devices, such as the MFCL above that use a magnetic field as one of the means to achieve superconductor quenching, are therefore critical in the operation of such devices. The applied magnetic field, referred herein as trigger magnetic field, should be generated with sufficient strength to cause quenching with a high degree of magnetic field uniformity. The compactness of the apparatus that generates such magnetic field should also he a major design consideration in order to reduce the overall size of a current limiting device. Generating a uniform trigger magnetic field is desirable since uniform quenching of a superconductor can significantly reduce the impact of xe2x80x9chot spotsxe2x80x9d (local regions of non-uniformity that may cause local heating or burn out during quench) on the thermal stability and reliability of the superconductor components.
Briefly, in accordance with one embodiment of the present invention, a tubular-configured superconductor element is arranged within a concentric outer coil to generate a magnetic field during current limiting. This tubular-configured superconductor has an axial length that is a single straight tube, a bifilar coil arranged in a tubular form, or multiple pieces of superconductor in low or non-inductive forms (tube, bar, rod, tape, etc.) electrically connected in series or in parallel and arranged in a tubular configuration. The outer coil has a fixed number of turns disposed closely adjacent to the outside diameter of the superconductor, which extend beyond the ends of the superconductor. The superconductor is situated within the outer coil in such a manner and the extension of the coil over the ends of the superconductor such that the magnetic field vector generated by the current flowing through the coil is oriented parallel to the length of the superconductor. A foil may replace the outer coil and is disposed around the superconductor in the same manner as the outer coil. In addition, a foil or a long wire carrying current that generates additional magnetic fields may be disposed inside the tubular-configured superconductor. The foil is disposed concentrically with the tubular-configured superconductor, while the wire is disposed along the center axial line of the tubular-configured superconductor. The length of the foil or the wire extends beyond the ends of the superconductor such that the magnetic field vector generated by the current in the foil or wire is tangent to the circumference of the superconductor and is perpendicular to the length of the superconductor, concurrently.
In an alternative embodiment of the present invention, a rod-type superconductor element is disposed inside a trigger coil that generates a uniform magnetic field during current limiting. The trigger coil has a fixed number of turns disposed closely adjacent the outer diameter of the superconductor, and the coil extends beyond the ends of the superconductor. The trigger coil is disposed along the superconductor in such a manner, and the extension of the coil over the ends of the superconductor such that the magnetic field vector generated by the current flows through this coil along the superconductor length and is oriented parallel to the current that flows through the superconductor. Such an arrangement is also applicable to superconductors having other low or non-inductive configurations.
In a further alternative embodiment of the present invention a method is provided for generating a uniform magnetic field applied to a no or low-inductance superconductor. The method consists of generating a uniform magnetic field by axially disposing a current-carrying coil on the outside of the superconductor, and by extending the length of the coil to beyond the ends of the superconductor. The coil is arranged such that the inner length extends beyond the ends of the superconductor such that the magnetic field vector generated by the coil is oriented parallel to the length of the superconductor. The coil may be replaced by a foil. In this case, the magnetic field vector is oriented perpendicular to the length of the superconductor. The method further consists of generating an additional uniform magnetic field by axially disposing a current-carrying foil or wire inside the tubular-configured superconductor having a low-inductance. The length of the foil extends beyond the ends of the superconductor such that the magnetic field vector generated by the current in the foil along the inner surface of the superconductor is tangent to the circumference of the superconductor and is perpendicular to the length of the superconductor, concurrently.